The present invention relates generally to rolling mills and particularly to the detection of instability in a mill that results in a phenomenon known as "chatter".
Individual stands of a rolling mill comprise a rigid housing and a stack of rolls rotationally supported in the housing by bearing chocks. Chatter is vibration occurring within a given stand and housing, which vibration may include a generally up-and-down linear motion of one or more of the rolls, a shifting of the work rolls in a stand in the direction of rolling and in the opposite direction, a torsional or rotational displacement of the rolls, or any combination of these motions.
Roll vibration results in visual parallel marks being rolled into the upper surface of the workpiece being rolled, the marks extending crosswise of the workpiece, i.e., crosswise to the direction of rolling. The parallel marks are a general waviness in the workpiece surface, which waviness distorts the reflection of light from the surface and distracts from its otherwise mirror-like surface finish. In addition, chatter marks are typically cyclic, with wavelengths of less than four inches and amplitudes less than 0.001 inch.
Further, coolants unevenly applied to the rolls of a rolling mill during the process of reducing the thickness of a workpiece can leave unwanted parallel marks on a surface of the workpiece. Such marks are uneven surface variations that effect the aesthetics of an otherwise bright surface.
Presently, the measurement of chatter and coolant marks on rolled strip has been subjective, based upon the impressions and opinions of experienced viewers, using the unaided eye, and influenced by ambient light and background reflections.
Chatter can also be a cyclic change in strip thickness or gauge. Such a deviation cannot be detected by the human eye.
Attempts have been made to objectively measure chatter by using small (six inch) samples of metal material and moving them individually past a contacting probe to measure the profile of the samples' surfaces. The samples were too small to provide an adequate analysis of surface conditions, and the probe, as a surface contacting device, was sensitive to surface contamination. Also, the probe did not provide data which related to what the human eye can see.
Another condition of a rolled metal strip, sheet and/or plate that requires control in the rolling process is known as the "profile" of the sheet thickness in the direction crosswise to the rolling direction. It is necessary to maintain profile as close as possible to a target value to guarantee good performance of the metal in finishing operations, such as cold rolling and leveling. Profile measurement is difficult because of the accuracy and precision required. The control of profile to .+-.0.01% on a 0.125 inch metal thickness requires the measurement to be repeatable within .+-.0.0000125 inch, which is in the neighborhood of surface roughness. If high performance contacting sensors are used, they are often degraded by the minute peaks and valleys in the rolled surface. Precise, non-contacting sensors lack the measurement range to track surface flatness for the full width of a metal sheet, i.e., any buckle or curvature in the sheet requires such sensors to move out of sensing range. Most non-contacting, analog-type probes have an accuracy of one part in 4,000. Therefore, if the metal flatness deviates by more than 0.1 inch, .+-.0.0000125 inch measurement is out of range for such probes.